The invention relates to the art of controls of hydroelectric power plants and more particularly to a new and improved system for the control of the draft tube water level when a turbine-generator is used as a synchronous condenser.
When a hydroelectric turbine-generator is used as a synchronous condenser, the high pressure water used to drive the unit as a generator is shut off, but the circuit breaker connecting it to the utility's electrical grid remains closed. To keep the power consumption of the turbine-generator at a minimum it is necessary to insure that the turbine is spinning in air above the water level of the draft tube at all times. The turbine elevation is often below the river or tailrace level. In this situation air pressure in the turbine cavity must be established and maintained at all times during condensing operation. This air pressure will gradually dissipate because of packing leakage. Therefore, after initial establishment of the air cushion in the turbine cavity, it must be periodically replenished with more air.
Maintenance of the correct water level in the draft tube is critical because power consumption swings, electric power system instability, and reduced plant efficiency will result from the loss of the draft tube air cushion.
All methods currently in use to control the draft tube water level have one thing in common. The process is controlled by devices deriving their logic from the draft tube or the turbine cavity. These methods sense the water level, or the pressure, directly at the draft tube or in the turbine cavity. These methods are not always dependable and require frequent maintenance. It would be highly desirable to provide a dependable and simple control system to maintain the water level in the draft tube.
The subject invention uses either the power or current consumption parameters of the generator for control of air injection into the turbine cavity thereby controlling draft tube water level.